I never really needed a micrometer Ted but the figures give an idea of the sheet's heft. My sheet is from a stack of 25 pre-cut and bought from the stationery shop which gives it as High White Ultra Smooth Card 200µ, 160gsm. The 275µ, 225gsm next model up won't go through either of my USB printers and absolutely none of my offline portable manual letterpress typewriters.

I use boxes like that for shipping stuff, and they are quite light yet sturdy! When properly packed and filled with shock-absorbing styrofoam or air bags, they can be sent safely through the FedEx system. Of course, those are made out of higher grade, corrugated cardboard, but the design is the same.